Method and device for analysing a sample using a resonant support, illuminated by infrared radiation

ABSTRACT

A method for analysing a sample uses a resonant support. The sample extends on the support having a surface on which a plurality of separated photonic crystals extend. The sample extends between a light source and the crystals, wherein a resonance wavelength is associated with each crystal addressing the analyte and the wavelengths of the crystals define a resonance spectral band extending between 2 μm and 20 μm. The transmission or reflection of light by each crystal addressing the analyte is maximum at the associated resonance wavelength. The method includes illuminating the support by the light source, the light source emitting an illumination lightwave defining an illumination spectral band which at least partially covers the resonance spectral band, such that a plurality of crystals are simultaneously illuminated; acquiring an image of the support, and then determining the presence of the analyte in the sample from the image.

TECHNICAL FIELD

The technical field of the invention is the analysis of a sample, the sample being deposited on a holder comprising photonic crystals the spectral properties of which in transmission or reflection comprise a resonant wavelength, the latter undergoing a variation under the effect of the presence of an analyte.

PRIOR ART

Use of infrared detection is conventional in the field of analysis of samples. Specifically, under the effect of infrared illumination, it is possible to detect a signature of a sample, from which signature information relating to the chemical composition of the sample may be obtained.

In the biology or health field, for example, the characterization of samples by infrared spectral imaging is now a technology that has been thoroughly described in the literature with respect to its applications in diagnostics, and in particular in the field of histopathology. It allows biomolecular information relating to cells or tissues to be obtained, without labeling. This method is based on a spectral signature of a sample vis-à-vis a pathology. When a light beam passing through the sample has a wavelength corresponding to an energy between two levels of molecular vibration, some of the beam is absorbed. Thus, via absorption spectrometry, it is possible to estimate a spectral absorbance of the sample, allowing information to be obtained on the molecular composition of the latter. The spectrum of the absorbance is thus a molecular signature of the sample.

However, in this type of method, it is necessary to scan the sample with a laser beam. The spatial resolution of the measurement is therefore dependent on the size of the laser beam. Hence, when it is desired to obtain spatially resolved spectral information, the laser beam must be narrow. As a result, an analysis of an area of sample of a few mm² or a few cm² takes a long time. In addition, the instrumentation associated with infrared spectral imaging is complex and expensive.

Outside the field of biological samples, infrared detection is conventional in the field of gas detection. This is for example the case with NDIR gas sensors (NDIR standing for non-dispersive infrared). In this type of sensor, a gas is illuminated by an infrared light source, in a spectral band of illumination corresponding to a spectral band of absorption of a gas species. A photodetector detects the light after it has propagated through the gas. The amount of light detected by the photodetector, in the spectral band of absorption, is representative of the attenuation of the light by the gas species. From the amount of light detected, it is possible to estimate an amount of the gas species in the gas.

The publication Andreas Titl et al. “Imaging based molecular barcoding with pixelated dielectric metasurfaces”, Science, vol. 360, n° 6393, 2018 Jun. 8, pp 1105-1109, describes a holder comprising resonant photonic crystals, allowing profiles of the absorption of infrared radiation by proteins to be obtained.

The document US2004/155309 describes use of metal structures that induce a plasmon resonance, to analyze samples.

The document Cheng F. et al. “Tuning asymmetry parameter of Fano resonance of spoof surface plasmons by modes coupling”, Applied Physics Letters, vol 100, n° 3, 2012 Mar. 26 describes metal structures that induce a plasmon resonance. Each structure consists of two patterns that are offset from each other, the offset being set.

The inventors have designed an analyzing device that is easy to use, that does not require complex instrumentation, such as a spectrometer, and that may be employed to study the composition of a sample.

SUMMARY OF THE INVENTION

A first subject of the invention is a method for analyzing a sample, the sample being liable to comprise an analyte, the sample lying on a resonant holder, the resonant holder comprising a surface on which lie a plurality of photonic crystals, which are separate from one another, such that the sample lies between a light source and the photonic crystals, the photonic crystals being such that:

-   -   one resonant wavelength is associated with each photonic         crystal, the resonant wavelengths of the photonic crystals         defining a spectral band of resonance, lying between 2 μm and 20         μm;     -   the transmission or reflection of light by each photonic crystal         is maximum at the resonant wavelength associated with it;         the method comprising the following steps:     -   a) illuminating the resonant holder with the light source, the         light source emitting an illuminating light wave that defines a         spectral band of illumination that at least partially covers the         resonance spectral band, such that a plurality of photonic         crystals are simultaneously illuminated;     -   b) acquiring, with an image sensor, an image of the resonant         holder, which image is referred to as the measurement image, the         image comprising different regions of interest, each region of         interest being optically coupled to one photonic crystal such         that each region of interest represents an intensity transmitted         or reflected by each photonic crystal, while the resonant holder         is illuminated;         then determining whether the analyte is present in the sample on         the basis of the measurement image.

Preferably, two different regions of interest are optically coupled to two different photonic crystals.

According to one embodiment, the method also comprises:

-   -   c) taking into account a reference image, the reference image         being representative of an image acquired by the image sensor,         when the resonant holder is illuminated in the spectral band of         illumination, in a reference configuration, in the absence of         analyte;     -   d) comparing the measurement image, acquired in step b), with         the reference image, taken into account in step c);     -   e) based on the comparison, determining whether the analyte is         present in the sample.

According to one embodiment, a plurality of photonic crystals are aligned in a row parallel to a longitudinal axis, such that the resonant wavelength respectively associated with a photonic crystal gradually increases, or decreases, along the longitudinal axis. The resonant holder may comprise various rows of photonic crystals parallel to one another, and wherein the photonic crystals form columns, parallel to a lateral axis, such that the photonic crystals of a given column have the same resonant wavelength. The longitudinal axis and the lateral axis are secant. The longitudinal axis is preferably perpendicular to the lateral axis.

Preferably, the photonic crystals lie in a holder plane, the illuminating light wave propagating to the resonant holder parallel to a propagation axis that is perpendicular or substantially perpendicular to the holder plane.

According to one embodiment, prior to step a), the method comprises a step of forming the reference image, comprising:

-   -   illuminating the resonant holder with the light source, in the         spectral band of illumination, without analyte between the light         source and the resonant holder;     -   acquiring an image of the resonant holder with the image sensor,         the image thus acquired forming the reference image.

The reference image may be an image obtained by:

-   -   illuminating, in the spectral band of illumination, a reference         holder that is considered to be representative of the resonant         holder illuminated in step a), the amount of analyte between the         reference holder and the light source being considered to be         zero;     -   forming an image of the reference holder, the image thus formed         corresponding to the reference image.

According to one embodiment, the resonant holder comprises reference photonic crystals,

-   -   the spectral band of resonance of which is not modified by the         analyte;     -   or the spectral band of resonance of which corresponds to a         wavelength not absorbed by the sample;         in which case the reference image is an image of the reference         photonic crystals when they are illuminated in the spectral band         of illumination.

The reference image and the measurement image may form two distinct parts of the same image acquired by the image sensor.

According to one embodiment,

-   -   the sample is a liquid;     -   the resonant holder makes contact with the sample;     -   the resonant holder comprises photonic crystals addressing the         analyte, which are configured to make contact with the analyte;     -   the measurement image comprises regions of interest that are         associated with the analyte, and that are optically coupled to         the photonic crystals addressing the analyte;     -   under the effect of contact with the analyte, the resonant         wavelength of the photonic crystals addressing the analyte         undergoes a spectral variation;     -   the illumination being such that the illumination intensity         varies, in the spectral band of resonance, according to a         spectral illumination function.

According to this embodiment, for each photonic crystal addressing the analyte, the resonant wavelength may depend on a refractive index of the sample, at an interface between the sample and the photonic crystal, the refractive index varying as a function of the amount of analyte making contact with the photonic crystal.

According to this embodiment, step e) may comprise the following sub-steps:

-   -   i) on the basis of the measurement image, determining a profile,         referred to as the measurement profile, of the intensity of the         regions of interest associated with the analyte;     -   ii) on the basis of the reference image, determining a profile,         referred to as the reference profile, representative of the         intensity, in the absence of analyte or in the presence of a         known amount of analyte in the sample, of the regions of         interest associated with the analyte;     -   such that whether the analyte is present is determined depending         on a shift of the measurement profile with respect to the         reference profile.

According to this embodiment, step e) may comprise the following sub-steps:

-   -   i) in the measurement image, determining a position, referred to         as the measurement position, of a region of interest exhibiting         a maximum intensity value among the regions of interest         associated with the analyte;     -   ii) in the reference image, determining a position, referred to         as the reference position, of a region of interest exhibiting a         maximum intensity value among the regions of interest associated         with the analyte;         such that the presence of the analyte is determined depending on         a shift of the measurement position with respect to the         reference position.

According to this embodiment, step e) may comprise estimating an amount of analyte in the sample, depending on the comparison between the measurement image and the reference image. The amount of analyte may be estimated depending on:

-   -   the shift of the measurement profile with respect to the         reference profile;     -   or the shift of the measurement position from the reference         position.

The amount of analyte may notably be a concentration of analyte.

The method may be such that:

-   -   each photonic crystal comprises holes produced through a thin         layer;     -   the resonant wavelength of each photonic crystal depends on the         radius or on the diagonal of the holes, and on the spacing         between the holes, and possibly on the refractive index of the         sample.

Each photonic crystal may comprise:

-   -   first holes that have a first dimension, the first dimension         being a radius or a diagonal, the first holes defining a first         periodic pattern;     -   second holes that have a second dimension, the second dimension         being strictly smaller than the first dimension, the second         holes defining a second periodic pattern;     -   the second pattern and the first pattern are offset with respect         to each other, parallel to the surface of the resonant holder,         by a spatial offset; such that the resonant wavelength         associated with each photonic crystal depends on the first         dimension, on the second dimension, and on the spatial offset.

According to one embodiment:

-   -   photonic crystals are aligned parallel to a longitudinal axis;     -   the resonant wavelengths respectively associated with two         adjacent photonic crystals are offset by a discretization pitch         comprised between 1 nm and 50 nm, and preferably between 1 nm         and 10 nm.

According to one embodiment, photonic crystals addressing a given analyte are covered with a functionalization layer conducive to selective capture of the analyte on the photonic crystals, for example by grafting.

According to this embodiment, the resonant holder may comprise:

-   -   photonic crystals of rank 1 that are configured to capture a         first analyte;     -   photonic crystals of rank j that are configured to capture a         j^(th) analyte that is different from the analytes captured by         the photonic crystals of rank lower than j.         j is a non-zero natural integer.

According to this embodiment:

-   -   the photonic crystals of the same rank may be aligned parallel         to a longitudinal axis;     -   and the photonic crystals having the same resonant wavelength in         the absence of captured analyte may be aligned parallel to a         lateral axis, the lateral axis and the longitudinal axis not         being parallel.

The resonant holder may be placed between the light source and the image sensor, such that, in step b), each region of interest formed in the measurement image is representative of an intensity transmitted by the photonic crystal to which said region of interest is optically coupled.

According to one embodiment, no image-forming optics are placed between the image sensor and the resonant holder;

According to one embodiment, an optical system, such as a lens or objective, is placed between the image sensor and the resonant holder, the optical system conjugating the image sensor with the resonant holder.

According to one embodiment, the resonant holder bounds a half-space, comprising the light source, and the image sensor is placed in the same half-space as the light source, such that each region of interest formed in the measurement image is representative of an intensity reflected by the photonic crystal to which said region of interest is optically coupled.

According to one embodiment, the sample is placed between the light source and the resonant holder, the analyte being able to absorb the illuminating light wave, emitted by the light source, in a spectral band of absorption, the method being such that the spectral band of absorption comprises at least one absorption wavelength corresponding to a resonant wavelength of a photonic crystal. The method comprises, on the basis of the measurement image:

-   -   determining at least one photonic crystal, the resonant         wavelength of which corresponds to the absorption wavelength;     -   identifying the resonant wavelength of said photonic crystal;     -   determining whether the analyte is present on the basis of the         resonant wavelength of the photonic crystal thus determined.

The method may then comprise:

-   -   on the basis of the measurement image, estimating an absorption         light intensity, transmitted or reflected by at least one         photonic crystal the resonant wavelength of which is comprised         in the spectral band of absorption;     -   estimating a reference light intensity, transmitted or reflected         by said photonic crystal, in the absence of analyte;     -   estimating an amount of analyte on the basis of the absorption         light intensity and of the reference light intensity.

The sample may be a gas or a liquid or a solid.

Another subject of the invention is a device for analyzing a sample, comprising a light source, an image sensor and a resonant holder, which is placed between the light source and the image sensor, such that the image sensor is configured to acquire an image of the resonant holder, the resonant holder being intended to be placed in contact with a sample, or facing a sample, the resonant holder comprising an area containing photonic crystals, which are separate from one another, the resonant holder being such that:

-   -   a resonant wavelength is associated with each photonic crystal,         such that the transmission or reflection of the light, by each         photonic crystal, is maximum at the resonant wavelength with         which it is associated, the resonant wavelengths of the photonic         crystals defining a spectral band of resonance, lying between 2         μm and 20 μm;     -   the light source is configured to simultaneously illuminate         various photonic crystals, in a spectral band of illumination         that at least partially covers the spectral band of resonance;     -   at least two photonic crystals are configured to make contact         with or to face the same analyte, said photonic crystals being         associated with two different resonant wavelengths, the resonant         wavelength associated with each photonic crystal depending on an         amount of analyte in contact with the photonic crystal.

According to one embodiment:

-   -   each photonic crystal comprises holes produced through a thin         layer;     -   the resonant wavelength of each photonic crystal depends on the         radius of and on the spacing between the holes, and on the         refractive index of the thin layer and possibly on the         refractive index of the sample.

According to one embodiment, each photonic crystal comprises:

-   -   first holes that have a first dimension, the first dimension         being a radius or a diagonal, the first holes defining a first         pattern;     -   second holes that have a second dimension, the second dimension         being strictly smaller than the first dimension, the second         holes defining a second periodic pattern;     -   the second pattern and the first pattern are offset with respect         to each other, parallel to the surface of the holder, by a         spatial offset;         such that the resonant wavelength associated with each photonic         crystal depends on the first dimension, on the second dimension,         and on the spatial offset.

According to one embodiment, the photonic crystals that are configured to make contact with the same analyte are aligned parallel to the same longitudinal axis, the resonant wavelengths respectively associated with two adjacent photonic crystals being offset by a discretization pitch comprised between 1 nm and 10 nm or between 1 nm and 50 nm.

According to one embodiment, photonic crystals, addressing an analyte, are configured to selectively capture the same analyte. They may notably be covered with a functionalization layer of interest, conducive to a selective capture of the analyte.

According to one embodiment, the analyzing holder comprises

-   -   photonic crystals of rank 1, which are configured to selectively         capture a first analyte;     -   photonic crystals of rank j, which are configured to selectively         capture a jth analyte that is different from the analytes of         rank lower than j.

According to one embodiment,

-   -   the photonic crystals of the same rank are aligned parallel to a         longitudinal axis;     -   and the photonic crystals associated with the same resonant         wavelength, in the absence of analyte, are aligned parallel to a         lateral axis, the lateral axis and the longitudinal axis not         being aligned.

According to one embodiment, the photonic crystals lie in a holder plane, and the light source is configured to emit, in the spectral band of illumination, an illuminating light wave that propagates to the holder along a propagation axis that is perpendicular or substantially perpendicular to the holder plane.

The plane of the holder may lie parallel to a detection plane defined by the image sensor.

The device may have one of the features described with reference to the first subject of the invention.

The invention will be better understood on reading the description of the examples of embodiment presented, in the remainder of the description, with reference to the figures listed below.

FIGURES

FIG. 1A is a first example of a device according to the invention.

FIG. 1B shows a second example of a device according to the invention.

FIG. 1C is a third example of a device according to the invention.

FIG. 1D is a detail of the resonant holder shown in FIGS. 1A, 1B and 1C.

FIGS. 2A to 2H schematically show the operation of the invention according to a first embodiment.

FIG. 2A shows an example of arrangement of the image sensor with respect to the resonant holder.

FIG. 2B schematically shows photonic crystals addressing the same analyte and distributed in the same row.

FIG. 2C shows an example of a spectral intensity function that the light source follows, in a spectral band of resonance defined by the resonant wavelengths of the photonic crystals formed on the resonant holder.

FIGS. 2D, 2E and 2F illustrate an intensity transmitted by each photonic crystal, respectively:

-   -   in the absence of analyte;     -   in the presence of a small amount of analyte captured by the         resonant holder;     -   in the presence of a large amount of analyte captured by the         resonant holder.

FIG. 2G shows profiles, along the longitudinal axis, of the intensity transmitted by the photonic crystals shown in FIG. 2D (dashed line), FIG. 2E (solid line) and FIG. 2F (dash-dotted line).

FIG. 2H schematically shows the main steps of a method according to the invention.

FIG. 3A shows a superposition of a resonant holder and of an image sensor of a device according to the invention, the invention operating in an embodiment that is referred to as two-dimensional.

FIGS. 3B and 3C illustrate an intensity transmitted by each photonic crystal in the presence of analytes captured by the resonant holder and in the absence of analyte captured by the holder, respectively.

FIG. 4A shows a variation of the resonant wavelength in transmission of photonic crystals as a function of wavelength.

FIG. 4B shows a detail of a resonant holder, the resonant holder comprising holes distributed with a double period.

FIG. 4C shows a variation in the resonant wavelength as a function of the arrangement of the holes of a photonic crystal such as shown in FIG. 4B.

FIG. 4D shows a modeled variation in the reflectance of a photonic crystal as a function of wavelength.

FIGS. 5A and 5B are examples of two different arrangements of holes forming photonic crystals.

FIGS. 6A to 6M illustrate steps for fabricating a resonant holder.

FIG. 7A shows a variation in the spectral transmission of a photonic crystal illuminated by a polychromatic light beam, at normal incidence, as a function of the illuminating wavelength. FIG. 7A was obtained experimentally.

FIG. 7B is a comparison of resonant wavelengths in reflection, measured experimentally and modeled, for various photonic crystals.

FIG. 8A shows one preferred embodiment of the invention.

FIG. 8B schematically shows the directions of polarization of polarizers described with reference to FIG. 8A.

FIGS. 9A and 9B are images of a resonant holder illuminated according to a spectral illumination function, and brought into contact with water and ethanol, respectively.

FIG. 9C is a comparison of horizontal profiles formed in FIGS. 9A and 9B, respectively.

FIGS. 10A to 10D schematically show images obtained according to a second embodiment.

FIGS. 10E and 10F schematically show profiles of an intensity of the light transmitted by photonic crystals in the configurations illustrated in FIGS. 10A and 10D, respectively.

DESCRIPTION OF PARTICULAR EMBODIMENTS

FIG. 1A shows an example of a device 1 for analyzing a sample according to the invention. The device 1 comprises a light source 10, which is configured to emit an illuminating light wave 11 that propagates to a sample 20. The illuminating light wave propagates parallel to a propagation axis Z, in a spectral band of illumination Δλ. The sample 20 is placed on a holder 15, which is referred to as the resonant holder. The sample 20 may comprise a gas, or a liquid, or a solid, comprising at least one analyte 21. The sample is placed between the light source 10 and the resonant holder 15.

By analyte, what is meant is a chemical or biological species the presence, and possibly an amount, of which in the sample it is desired to determine. The analyte 21 may, for example, be a chemical molecule, a protein, a peptide, an antibody, an antigen, a fragment of a nucleotide sequence, or a particle. By particle, what is meant for example is a biological cell, a droplet that is insoluble in a medium, or a nanobead. It may also be a question of a microorganism, a bacterium for example, a yeast or a microalgae. Preferably, a particle has a diameter, or is inscribed in a diameter, smaller than 20 μm, or even than 10 μm or than 5 μm.

When the sample is a gas, the analyte is a gas species with a spectral band of light absorption Δλ_(a) comprised in the spectral band of illumination Δλ. The gas species may for example be CH₄, CF₄, CO₂, NO, or NH₃. Generally, the gas species exhibits absorption that is considered to be significant in the spectral band of illumination. The case of a gas sample is addressed in the second embodiment.

When the sample is a solid, it is sufficiently transparent, in at least one part of the spectral band of illumination Δλ, so as to allow light to propagate through the sample. It may for example be a question of a thin slide of biological tissue obtained from a sample of biopsy or smear type. It is for example a question of a pathology slide. It may also be a question of a dry extract obtained by drying a bodily liquid, of a thin slide of dried blood for example. The analyte may then be a molecule, or one portion of a molecule, or of a molecular bond, exhibiting a spectral band of absorption Δλ_(a) in the spectral band of illumination Δλ. The case of a solid sample is addressed in the second embodiment.

By resonant holder, what is meant is a holder one portion of which is able to resonate, so as to transmit or to reflect a maximum light intensity at a resonant wavelength. The structure of the resonant holder is described below.

The sample 20 may take the form of a drop deposited on the resonant holder 15. It may also be a question of a liquid or gas confined in a fluidic chamber 18 associated with the resonant holder 15. The resonant holder may for example consist of a wall of the fluidic chamber 18 in which the sample lies. The fluidic chamber 18 is preferably transparent in the spectral band of illumination Δλ. It may for example comprise or be made of materials that are transparent in the infrared: it is for example a question of silicon, of germanium, of calcium fluoride (CaF₂) or of barium fluoride (BaF₂), or of zinc sulfide (ZnS) or of a chalcogenide.

The light source 10 may be monochromatic or polychromatic. The illuminating wave 11 lies in a spectral band of illumination Δλ. The spectral band of illumination lies between 2 μm and 20 μm. Preferably, the spectral band of illumination extends between a minimum wavelength and a maximum wavelength, the minimum wavelength being strictly longer than 1.5 μm, or even 2 μm. Thus, the spectral band of illumination may lie in the near infrared (typically between 2 μm and 3 μm), or in the mid-infrared (typically between 3 and 8 μm), or in the far infrared (typically up to 20 μm).

The light source 10 may be formed by a heated filament, forming a light source the spectral emission of which is close to that of a black body. The light source may be formed by a laser. It may be a question of a wavelength-tunable laser, and for example of a quantum cascade laser (QCL). The light source may comprise a plurality of elementary QCL sources, emitting in various spectral bands of illumination, respectively.

In a first embodiment, described with reference to FIGS. 2 to 9, the width of the spectral band of illumination Δλ is preferably smaller than 200 nm, or even smaller than 100 nm or than 10 nm. In a second embodiment, described with reference to FIGS. 10A to 10F, the width of the spectral band of illumination Δλ may be a few μm.

Preferably, the light source 10 is placed at a distance A from the resonant holder 15, such that the light wave 11 reaches the latter in the form of a plane wave. A collimating optical element, known to those skilled in the art, may be placed between the light source 10 and the sample 20, so as to form a plane light wave 11.

The device 1 also comprises an image sensor 30 that is sensitive to the infrared. The image sensor is preferably a pixelated sensor, comprising pixels 31 arranged in a matrix array. The pixels of the image sensor 30 define a detection plane P₃₀. In this example, the image sensor is formed by a matrix array of bolometers, each bolometer of the matrix array having a spectral band of detection comprised between 2 μm and 20 μm. Each bolometer forms one pixel. In the examples described below, each pixel is formed by a vacuum-encapsulated bolometer.

Preferably, the detection plane P₃₀ is placed perpendicular to the propagation axis Z, or substantially perpendicular to the latter. By substantially perpendicular, what is meant is perpendicular to within an angular tolerance of +20°, or preferably ±10°, or even ±5°. Thus, the illuminating wave 11 emitted by the light source 10 reaches the holder at a normal incidence, to within the angular tolerance.

Preferably, the resonant holder 15 defines a holder plane P₁₅. The holder plane P₁₅ lies perpendicular to the propagation axis Z, or substantially perpendicular to the latter. The resonant holder extends along a longitudinal axis X and a lateral axis Y. The axes X and Y are coplanar with the holder plane. They are secant, and preferably perpendicular.

Preferably, the detection plane P₃₀ extends parallel to the holder plane P₁₅.

An important aspect of the invention is that the resonant holder 15 comprises nanostructured elementary zones that are separate from one another, each elementary zone forming one photonic crystal 16 _(k). Thus, the resonant holder comprises photonic crystals 16 _(k), which are different from one another, and which are spaced from one another. The index k is an integer strictly comprised between 1 and K, K corresponding to the number of photonic crystals formed on the resonant holder 15.

Each photonic crystal 16 _(k) has a resonant wavelength λ_(k) that is specific thereto. At the resonant wavelength λ_(k) that is associated with it, each photonic crystal 16 _(k) exhibits a peak in light transmission or reflection. Each crystal thus has a resonant wavelength in reflection and a resonant wavelength in transmission.

The image sensor 30 is configured such that the pixels 31 are divided into groups of pixels 32 _(k), the pixels 31 belonging to the same group of pixels 32 _(k) being optically coupled to the same photonic crystal 16 _(k).

By group of pixels, what is meant is a pixel or a set of pixels that are adjacent to one another and optically coupled to the same photonic crystal.

By optically coupled, what is meant is that the pixels 31 of the same group of pixels 32 _(k) collect light the intensity of which is at least 80% or 90% due to light propagating through the photonic crystal 16 _(k).

In the example shown in FIG. 1A, the distance d between each photonic crystal 16 _(k) and pixel group 32 _(k) that is respectively associated with it, is 500 μm. This distance is preferably comprised between a few tens of μm and 1 mm. A small distance improves the optical coupling between each photonic crystal 16 _(k) and pixel group 32 _(k) associated with it. The configuration shown in FIG. 1A is a lensless imaging configuration, with no imaging optics interposed between the resonant holder and the image sensor 30. This is an inexpensive and compact configuration.

Alternatively, as shown in FIG. 1B, an optical system 12, for example an infrared lens or objective, is placed between the resonant holder 15 and the image sensor 30. The optical system 12 allows each photonic crystal 16 _(k) to be optically coupled to one group of pixels 32 _(k). Such an optical system 12 allows each diffraction grating 16 _(k) to be conjugated with one group of pixels 32 _(k).

The light source is placed at a distance A from the resonant holder, such that a plurality of photonic crystals are simultaneously illuminated by the light source. Thus, at least 3, and preferably at least 5 or 10 photonic crystals are simultaneously illuminated by the light source. The illumination of the resonant holder is preferably extensive, in the sense that the illuminated area is preferably larger than 1 mm², or even 1 cm². The illuminated area may correspond to the field of observation of the image sensor 30. In the lens-less configuration described with reference to FIG. 1A, the illuminated area corresponds to the area of the image sensor, i.e. to an area of the order of 35 mm².

The device 1 also comprises a processing unit 40, for example a microprocessor, configured to process and/or display images acquired by the image sensor 30. The processing unit may be connected to a memory 41, comprising instructions for implementing image-processing algorithms. The processing unit 40 is preferably connected to a screen 42.

The configurations schematically shown in FIGS. 1A and 1B are transmission configurations, in which the resonant holder 15 is interposed between the light source and the image sensor 30. Thus, the image sensor 30 detects a light wave 14 transmitted by the resonant holder 15. By transmitted, what is meant is propagating in the same direction as the incident light wave 11. In another configuration, illustrated in FIG. 1C, the holder plane P₁₅ separates space into two half-spaces. The image sensor 30 is located in the same half-space as light source 10. In the example shown in FIG. 1C, a half-silvered mirror 13 reflects toward the sample 20 the light wave 11 emitted by the light source 10, and transmits to the image sensor 30 a light wave 14′ reflected by the resonant holder 15. Such a configuration is usually designated by the expression “reflection configuration”. An optical system 12 allows each photonic crystal 16 _(k) to be optically coupled to one group of pixels 32 _(k) of the image sensor 30, as described above.

FIG. 1D shows a cross section, in a plane perpendicular to the holder plane P₁₅, of a detail of the resonant holder 15. Each photonic crystal 16 _(k), 16 _(k+1) is formed by holes 23 that are formed in a thin layer 22, and that open onto the surface of the latter. The thin layer 22 may be made from a dielectric or semiconductor. The holes 23 are arranged in a periodic pattern, which is preferably two-dimensional. During use of the resonant holder, the holes 23 are intended to be filled by the sample 20. It is preferable for the thin layer 22 to have a refractive index very different from the refractive index of the sample, so as to obtain a high index contrast between the sample 20, filling the holes 23, and the thin layer 22. Preferably, the refractive-index difference between the sample and the material forming the thin layer 22 is larger than 0.1, or even than 0.5. Thus, the refractive index of the thin layer 22 is preferably higher than 2, and more preferably higher than 3. For example, the thin layer may be made of silicon (n=3.48), or of silicon nitride Si₃N₄ (n=2) or of titanium oxide (TiO₂). When the spectral band of illumination extends beyond 5 μm, the thin layer 22 may comprise germanium. It may for example be made of a silicon/germanium mixture or of a chalcogenide.

Thus, the thin layer 22 comprises a plurality of resonant photonic crystals 16 _(k) that are separate from one another, and that each have a resonant wavelength λ_(k) in transmission or in reflection. The thickness of the thin layer 22 is adjusted depending on the spectral band of illumination. It is preferably comprised between 50 nm to a few μm. When the spectral band of illumination Δλ extends beyond 5 μm, the thickness of the thin layer may for example reach 1.5 μm to 3 μm. The holes 23 extend through the thin layer 22, about an axis perpendicular to the plane of the holder. The holes 23 may in particular extend about an axis parallel to the propagation axis Z.

In the holder plane P₁₅, the photonic crystals 16 _(k) have a diagonal or a diameter comprised between 10 μm and 500 μm, of 100 μm for example. Each photonic crystal is borne by a transparent or translucent membrane, formed by a layer 24, which may be made of silicon. Fabrication of the photonic crystals is described below, with reference to FIGS. 6A to 6M.

The spectral properties of the photonic crystals with respect to transmission and reflection of light may be determined using simulations performed via computer codes. Specifically, the properties of propagation of light in the photonic crystals stem from their specific periodic arrangement. These propagation properties may easily be modeled, by a person skilled in the art, on the basis of Maxwell's equations. In the remainder of this description, the modelling was carried out using the software package Rsoft and a method of RCWA type.

By photonic crystal, what is meant is a structure the refractive index of which varies periodically, on the wavelength scale, in one or more directions. In the examples described in this description, the photonic crystals are two-dimensional, this being a preferred configuration. The invention therefore takes advantage of the development of techniques for micro-structuring dielectrics, semiconductors or metals, allowing control of the interaction of electromagnetic waves in three-dimensional structures based on the arrangement of materials of various indices.

Each photonic crystal 16 _(k) transmits (or reflects) light according to a spectral transmission (or reflection) function describing a variation in a light intensity transmitted (or reflected) by the photonic crystal, as a function of wavelength. The exploitation of Fano resonance makes it possible to design compact resonant photonic crystals, which may be illuminated collectively by one illuminating light wave, at a normal incidence. This makes it possible to simultaneously illuminate, in a simple manner, photonic crystals distributed in one or two dimensions. The spectral transmission function of each photonic crystal exhibits a maximum, at a resonant wavelength in transmission. Analogously, the spectral reflection function exhibits a maximum, at a resonant length in reflection.

The spectral transmission or reflection function, and in particular the resonant wavelengths in transmission or in reflection, depend first of all on the structure of each photonic crystal, i.e. on the size and on the spatial arrangement of the holes 23 produced in the thin layer 22, to form the photonic crystal 16 _(k). In reflection or in transmission, the resonant wavelength λ_(k) also depends on the refractive-index contrast between the thin layer 22 and the sample, in the holes 23. The resonant wavelength also depends on the thickness of the thin layer 22. It also depends on the refractive index and on the thickness of the transparent layer 24.

FIGS. 2A to 2H illustrate the principle of a first embodiment of the invention. According to this embodiment, the sample is liquid. It makes contact with the holder 15, and notably contact with the photonic crystals 16 _(k). In this example, the configuration is a transmission configuration such as illustrated in FIG. 1A. FIG. 2A shows the resonant holder 15, and photonic crystals 16 _(k) distributed over the resonant holder. The image sensor 30 lies parallel to the resonant holder. Groups of pixels 32 _(k) have also been shown, each group of pixels being optically coupled to one photonic crystal 16 _(k). Each group of pixels 32 _(k) comprises at least one pixel. In the lensless-imaging configuration (see FIG. 1A), one group of pixels may be optically coupled to one photonic crystal by minimizing the distance d between the image sensor 30 and the resonant holder 15. When an optical system 12 is used, coupling is achieved via the optical system (see FIG. 1B or FIG. 1C), said optical system conjugating each group of pixels 32 _(k) with one photonic crystal.

Before it is brought into contact with the sample, the resonant holder 15 will have undergone a surface functionalization, in the holder plane P₁₅, such that each photonic crystal is able to capture a predetermined analyte. Surface functionalization is a concept known to those skilled in the art. It consists in adding a specific function to the surface to be functionalized, by nanostructuring, or by depositing a coating, or by adsorbing or grafting molecules with specific properties. In the present case, the surface functionalization confers, on each photonic crystal, a property enabling selective capture of an analyte. The analyte may be captured via formation of a covalent, hydrogen or electrostatic bond with the analyte and/or via grafting of the analyte with a ligand placed on the functionalized capturing surface. Following the functionalization, the holder plane P₁₅ is also a functionalized holder plane, i.e. one that has been functionalized to capture one or more analytes.

When the resonant holder is placed in contact with the sample, the sample fills the holes 23 of each photonic crystal 16 _(k). The spectral properties of each photonic crystal 16 _(k) with respect to transmission (or reflection) of the light are then governed by the structure of each photonic crystal (in particular the size and distribution of the holes 23) and the respective refractive indices of the sample 20 and of the thin layer 22.

In FIG. 2B, the photonic crystals 16 _(k) have been shown addressing the same analyte, i.e. they are configured to capture the same analyte. They are aligned in rows, parallel to the longitudinal axis X.

Because of the surface functionalization, when the desired analyte is present in the sample, the concentration of analyte captured by the holder increases after the latter has been brought into contact with the sample. This results in a local variation in the index of the sample, at the interface between the sample and the functionalized holder. Under the effect of such a variation in index, the spectral transmission (or reflection) properties of each functionalized photonic crystal change—in particular the resonant wavelength of the photonic crystals changes.

A noteworthy aspect of the invention is that the photonic crystals are dimensioned such that:

-   -   the respective resonant wavelengths of the photonic crystals,         and notably of photonic crystals addressing the same analyte,         are different from one another;     -   the spectral offset between two resonant wavelengths of two         adjacent photonic crystals is small and controlled.

All the resonant wavelengths of photonic crystals addressing the same analyte lie between a minimum resonant wavelength λ_(r,min) and a maximum resonant wavelength λ_(r,max). The latter bound a spectral band of resonance Δλ_(r). It is necessary for the spectral band of illumination Δλ of the light source to be, at least partially, included in the spectral band of resonance Δλ_(r). The spectral band of illumination Δλ may be the same as the spectral band of resonance Δλ_(r). It may be wider or narrower than the spectral band of resonance Δλ_(r). FIG. 2C shows the spectral band of illumination Δλ, the spectral band of resonance Δλ_(r) and the respective resonant wavelengths λ_(k=1), . . . λ_(k), . . . λ_(k=K) of the photonic crystals 16 _(k=1), . . . 16 _(k), . . . 16 _(k=K).

In the spectral band of resonance Δλ_(r), which is defined by the photonic crystals of the holder, the intensity of the light wave 11 emitted by the light source 10 is not constant. In the spectral band of resonance Δλ_(r), the intensity of the light wave 11 is variable, and follows a spectral illumination function ƒ. The spectral illumination function ƒ defines the intensity of the incident light wave 11 at various wavelengths of the spectral band of resonance, such that:

I(λ)=ƒ(λ)

where:

-   -   λ is a wavelength of the spectral band of resonance Δλ_(r);     -   I(λ) is the intensity of the illuminating wave 11, or         illumination intensity, at the wavelength λ.

It is important that the illumination function ƒ not be constant, i.e. that the intensity of the illuminating wave not be uniform in the spectral band of resonance Δλ_(r). The illumination function ƒ may thus be monotonic in the spectral band of illumination, and for example an increasing or decreasing function. It may also increase (decrease, respectively) to an extremum and then decrease (increase, respectively) from the extremum. When the light source is monochromatic, the illumination function forms a peak in the spectral band of resonance Δλ_(r).

In the preferred embodiment, two adjacent photonic crystals 16 _(k), 16 _(k+1) have resonant wavelengths that are offset by a known spectral offset dλ_(k). The spectral resolution of the method is dependent on the spectral offset dλ_(k). In absolute value, the spectral offset dλ_(k) is preferably smaller than 10 nm, and more preferably smaller than nm, or even than 2 nm. It corresponds to a spectral discretization pitch, with which the illuminating light wave 11 is discretized, as described with reference to FIG. 2G. Photonic-crystal structures allowing such an offset to be obtained are described below.

On capture of the analyte, the resonant wavelength of each photonic crystal changes, passing from a value λ_(ref,k), in the absence of analyte, to a value λ_(k) in the presence of analyte, with λ_(k)=λ_(ref,k)+δA (1). It will be noted that the variation in resonant wavelength δλ is the same for all the photonic crystals addressing a given analyte. In other words, the sensitivity of the resonant wavelength to variations in index is the same for all the various photonic crystals.

The change in resonant wavelength δλ is consecutive to the capture of the analyte, which generally results in an increase in the index at the interface between the resonant holder and the sample. The increase in index causes a variation in the resonant wavelength of each photonic crystal.

Thus, the invention is based on a measurement of a variation in resonant wavelength δA under the effect of capture of the analyte by photonic crystals addressing the same analyte.

The variation δλ in resonant wavelength is observed by taking into account a reference configuration, in which the captured amount of analyte is known. Preferably, in the reference configuration, the amount of analyte captured by each photonic crystal is zero. This is the case that will be considered in the remainder of the description. Alternatively, provision may be made for an embodiment in which in the reference configuration corresponds to a known captured amount of analyte.

The precision with which the variation δλ in resonant wavelength is estimated depends on the spectral offset dλ_(k) between two adjacent photonic crystals. The smaller the spectral offset dλ_(k), the better the resolution with which the variation δλ is estimated.

The variation in resonant wavelength δλ is determined by comparing a reference image I_(ref) of the resonant holder, in the reference configuration, with an image I of the resonant holder taken after capture of the analyte. The image I of the holder, after capture, is called the measurement image.

FIG. 2C shows a spectral illumination function ƒ, as mentioned above. The y-axis represents the intensity I(λ) of the illuminating light wave 11 as a function of wavelength λ. In the present case, the spectral illumination function has a maximum, comprised between the limits λ_(r,min), λ_(r,max) of the spectral band of resonance Δλ_(r). Alternatively, the spectral illumination function ƒ may have a minimum in the spectral band of resonance Δλ_(r). Irrespectively of whether it is a question of a minimum or a maximum, the extremum is preferably comprised in the middle of the spectral band of resonance Δλ_(r). Preferably, the extremum is located between the middle Δλ_(r)/2 of the spectral band of resonance Δλ_(r) and the maximum wavelength λ_(r,max).

FIG. 2D shows the intensities detected in each region of interest ROI_(k) of an image I_(ref) acquired by the image sensor, in the reference configuration, in the absence of analyte capture. Each region of interest ROI_(k) of the image corresponds to one group of pixels 32 _(k) optically coupled to one photonic crystal 16 _(k). Thus, each region of interest ROI_(k) is representative of a light intensity transmitted by each photonic crystal 16 _(k), at the resonant wavelength λ_(ref,k) of the latter.

The regions of interest ROI_(k) are distinct from one another, and aligned parallel to the photonic crystals 16 _(k). In FIGS. 2D to 2F, the intensities have been represented in grayscale, the darkest grayscale level corresponding to a maximum intensity value.

FIG. 2E shows the intensities of each region of interest ROI_(k) of an image, acquired by the image sensor, in the measurement configuration, after capture of the analyte by the photonic crystals. Each region of interest ROI_(k) is representative of a light intensity transmitted by each photonic crystal 16 _(k), at the resonant wavelength λ_(k) of the latter. Under the effect of the capture of an analyte, the resonant wavelength λ_(k) is different from the resonant wavelength λ_(ref,k) with λ_(k)=λ_(ref,k)+δλ.

Comparison of the measurement image I shown in FIG. 2E, and of the reference image I_(ref) shown in FIG. 2D, allows the effect of the variation in the resonant wavelength of each photonic crystal under the effect of the analyte capture to be seen.

On the basis of the measurement image and of the reference image, it is possible to form an intensity profile representative of a spatial distribution of the intensity of each region of interest ROI_(k) along the longitudinal axis X. FIG. 2G shows a reference profile (dashed line) formed on the basis of the reference image, and a measurement profile (solid line) formed on the basis of the measurement image. Use of such profiles facilitates the comparison of two images I and I_(ref) and the estimation of the variation δλ in the resonant wavelength of the photonic crystals under the effect of the analyte capture. In the representation of FIG. 2G, each profile is a profile interpolated from the discrete values of the intensities of the various regions of interest ROI_(k) of the image in question.

On account of the alignment of the photonic crystals along the same axis X, and of the small spectral offset dλ_(k) between two adjacent photonic crystals, each profile has approximately the same shape as the spectral illumination function. Thus, the resonant holder 15 allows spectral information to be converted into spatial information. It acts in the same way as a spectrometer. The spatial information corresponds to a position, along the axis X, of each region of interest ROI_(k). A spectral variation δλ, in the present case the variation in resonant wavelength affecting each photonic crystal, results in a spatial shift Δx of the profile, between the reference image and the measurement image. As the spectral offset dλ_(k) between two adjacent photonic crystals is set, the variation in resonant wavelength δλ may be estimated, using the expression:

$\begin{matrix} {{\delta\lambda} = {\sum\limits_{k}^{k + {\Delta k}}{d\lambda_{k}}}} & (2) \end{matrix}$

where:

-   -   Δk corresponds to the spatial variation, which is obtained by         comparing the measurement image and the reference image, and         which represents the number of regions of interest 32 _(k)         forming the observed spatial variation Δx;     -   Σ_(k) ^(k+Δk)dλ_(k) corresponds to the sum of the spectral         offsets between the photonic crystals respectively associated         with the regions of interest forming the observed spatial         variation Δx.

Preferably, the spectral offset dλ_(k) between two adjacent photonic crystals may be considered to be constant and equal to dλ. In this case, the preceding expression becomes:

δλ=Δk×dλ  (3)

The advantage of using a non-uniform spectral illumination function ƒ will now be understood. It facilitates a comparison of the intensity profiles in the reference configuration and in the measurement configuration, respectively, so as to allow the number Δk of regions of interest ROI_(k) by which the intensity profile shifts between the reference configuration and the measurement configuration to be estimated. In the example shown in FIGS. 2D and 2E, Δk=3. The offset dλ is known and is related to the geometry of the photonic crystals respectively associated with each region of interest, as described in the remainder of the description.

In contrast to certain prior-art devices that require a spectrometer to be used, the invention transfers the spectral splitting function to the resonant holder 15. This allows the use of a simple image sensor, which is clearly less expensive and complex to implement.

According to one embodiment, the light source is monochromatic, and may be tunable. The spectral band of illumination may then be narrower than or equal to the spectral resonant-wavelength offset dλ_(k) between two adjacent photonic crystals. In order to be able to estimate the variation δλ in resonant wavelength under the effect of the capture of the analyte, the spectral band of illumination then corresponds to a resonant wavelength of a single photonic crystal in the reference configuration, i.e. in the absence of analyte capture, and in the measurement configuration.

The light source may be monochromatic and tunable, so that the spectral band of illumination may be modified.

The reference image may be an image, of the sample, acquired before the capture. In this case, the reference image is taken at an initial time, at which the capture of the analyte by the holder is considered to be negligible. It may for example be a time close to the time at which the holder and the sample are brought into contact. The measurement image is then acquired after the reference image.

Alternatively, the reference image is an image obtained with a reference holder that is considered to be representative of the holder brought into contact with the sample. The reference holder is then brought into contact with a reference sample that is considered to be representative of the analyzed sample. The reference image is stored in memory. Such an embodiment assumes a good reproducibility in the fabrication of the holders.

Thus, the reference image, taken into account to estimate the variation in resonant wavelength, may be:

-   -   acquired by the image sensor, prior to the acquisition of the         measurement image;     -   or obtained using another holder, then stored in memory;     -   or acquired by the image sensor, simultaneously with the         measurement image, using the same resonant holder, the reference         image corresponding to the image of photonic crystals configured         not to capture the analyte. It is a question, for example, of         non-functionalized photonic crystals. In this case, one portion         of the image acquired by the image sensor forms the measurement         image: this corresponds to the regions of interest of the         acquired image representing the photonic crystals having         captured the analyte, or, more generally, making contact with         the analyte. Another portion of the image acquired by the image         sensor forms the reference image: it corresponds to the regions         of interest of the acquired image representing the photonic         crystals not having captured the analyte, or, more generally,         not making contact with the analyte. For example, the photonic         crystals functionalized to capture the analyte are aligned in a         first row, whereas the photonic crystals not configured to         capture the analyte are aligned in a second row, the two rows         being parallel to each other. The image of the first row forms         the measurement image, whereas the image of the second row forms         the reference image. Thus, from the same image acquired by the         image sensor, the measurement image and the reference image may         be obtained.

Regardless of the embodiment, the reference image is formed by illuminating the holder or the reference holder, in a spectral band of illumination Δλ and according to a spectral illumination function ƒ equal to those employed to acquire the measurement image. In other words, the reference image and measurement image of the employed holder(s) are obtained under the same illumination conditions.

FIG. 2F shows a measurement image in which the amount of analyte captured by the photonic crystals is greater than the amount corresponding to the image of FIG. 2E. This results in a larger variation in index at the interface between the photonic crystals and the sample, resulting in a larger variation in resonant wavelength δλ′. In this image, each region of interest ROI_(k) is representative of the intensity transmitted by the photonic crystal 16 _(k) with which it is associated, at a resonant wavelength λ′_(k) of the latter, with λ′_(k)=λ_(ref,k)+δλ′. In FIG. 2G, the corresponding profile is dash-dotted.

The method makes it possible to detect a presence of the analyte in the sample, this corresponding to the observation of a non-zero variation δλ in resonant wavelength. It also makes it possible to estimate an amount of analyte captured by the resonant holder, with an precision depending on the number of photonic crystals and on the spectral offset dλ_(k) between the resonant wavelength of two adjacent photonic crystals.

On the basis of the amount of analyte captured by the holder, it is then possible to estimate an amount of analyte in the sample, said amount possibly for example being expressed in the form of a concentration. A calibration, using samples the analyte concentration of which is known, allows a calibration function, relating the amount of analyte captured by the holder to the analyte concentration initially present in the sample, to be obtained.

FIG. 2H summarizes the main steps of the method.

Step 100: bringing the sample into contact with the resonant holder.

Step 110: illuminating the sample in the spectral band of illumination Δλ, according to the spectral illumination function ƒ.

Step 120: acquiring a measurement image I of the resonant holder.

Step 130: taking into account of a reference image I_(ref), the reference image possibly being an image of the holder acquired just after step 110, prior to the acquisition of the measurement image, this forming the optional step 115.

Step 140: comparing the measurement image I with the reference image I_(ref), so as to estimate a variation in the resonant wavelength δλ of the photonic crystals. It is notably a question of determining the number of photonic crystals (or the number of regions of interest) corresponding to the variation in resonant wavelength δλ. Knowing the spectral offset dλ_(k) between the resonant wavelengths of the photonic crystals, it is thus possible to determine the variation in resonant wavelength δλ.

Step 150: on the basis of the comparison, determining whether the analyte addressed by the photonic crystals is present in the sample, and possibly estimating an amount of analyte in the sample.

In the example shown in FIGS. 3A to 3C, the resonant holder comprises photonic crystals 16 _(k) distributed in J rows X₁ . . . X_(j) . . . X_(J). The photonic crystals of a given row X_(j) are configured to address a given analyte. Thus, the photonic crystals of the first row X₁ address a first analyte, and the photonic crystals of the j^(th) row X_(j) address a j^(th) analyte. The index j is a natural integer such that 1≤j≤J.

As described with reference to the first embodiment, the photonic crystals are distributed in columns Y₁, Y₂, Y_(k), Y_(K), such that the photonic crystals belonging to two adjacent columns Y_(k) have, in the reference configuration, the same resonant wavelength λ_(k,ref).

This configuration, which is referred to as the two-dimensional configuration, corresponds to the addition of one dimension to the embodiment described with reference to FIGS. 2A to 2H. The added dimension corresponds to the type of analyte. Such a configuration assumes that each row has undergone a different functionalization to the others, so that each analyte may be captured by one row, and preferably by only one row. Thus, the first row X₁ is functionalized so as to capture a first analyte. The j^(th) row X_(j) is functionalized so as to capture a j^(th) analyte, which is preferably different from the analytes addressed by the rows of lower rank.

It will be noted that the device allows a plurality of photonic crystals to be illuminated simultaneously, this making it possible to obtain exploitable information relating to various analytes from a single image acquired using a simple image sensor.

According to this embodiment, steps 100 to 150, which were described with reference to FIG. 2H, are repeated for each row X_(j) of the holder. FIGS. 3B and 3C schematically show a measurement image and a reference image, respectively. In these figures, the light source is monochromatic. Thus, only the photonic crystal the resonant wavelength of which corresponds to the spectral band of illumination forms a high-intensity spot in the region of interest associated with it in the image acquired by the image sensor.

In the reference image (FIG. 3C), the regions of interest corresponding, in each row, to the illuminating wavelength are aligned. Comparison of the measurement image (FIG. 3B) and of the reference image reveals a large variation in the resonant wavelength of the photonic crystals of the fourth row, a smaller variation in the resonant wavelength of the photonic crystals of the first row and 3^(rd) row, and zero variation in the resonant wavelength of the photonic crystals of the 2^(nd) row. This allows it to be concluded that the first analyte, the 3^(rd) analyte and the 4^(th) analyte are present in the sample, and that the second analyte is absent. By means of calibrations carried out for each analyte, each variation in resonant wavelength allows a concentration of the analytes considered to be present in the sample to be estimated.

According to one such embodiment, the device allows the presence of various analytes in the sample to be detected simultaneously, and the concentration thereof to be estimated.

FIG. 4A shows various modeled spectral transmission functions of various photonic crystals, each modeled photonic crystal having various geometric characteristics. In this figure, the x-axis corresponds to the wavelength, expressed in μm, whereas the y-axis represents the intensity of the transmitted light.

The modeling the results of which are shown in FIG. 4A was carried out for a preferred embodiment, in which each photonic crystal 16 _(k) is arranged in a structure as shown in FIG. 4B. Such a structure, referred to as the double-period structure, comprises:

-   -   first holes 23 ₁, of a first radius R₁, and distributed         according to a first periodic pattern P₁;     -   second holes 23 ₂, of a second radius R₂ strictly smaller than         the first radius R₁, and distributed according to a second         periodic pattern P₂. The second periodic pattern P₂ is, in this         example, similar to the first periodic pattern P₁, and offset         with respect to the latter.

In this example the first periodic pattern P₁ and the second periodic pattern P₂ define a rectangular mesh, of period a along X and 2a along Y, the rectangular mesh being reproduced in two orthogonal directions of translation. In this example, the two directions of translation correspond to the longitudinal axis X and to the lateral axis Y, respectively. The first periodic pattern P₁ and the second periodic pattern P₂ are interlaced in the sense that, in one direction of translation, in the present case the Y-direction, an alternation is observed between rows of first holes 23 ₁, parallel to the longitudinal axis X, and rows of second holes 23 ₂, also parallel to the longitudinal axis X. In this example, the period a₁ (along X) of the first periodic pattern P₁ is equal to the period a₂ (along X) of the second periodic pattern P₂, the common period of the two patterns being denoted a. The value of the period is typically between 100 nm and 5 μm. In this example, the period a is larger than 1 μm. Generally, if A is a wavelength belonging to the spectral band of illumination Δλ, the period a is comprised between λ/4 and λ.

One advantage of a double-period resonant structure is the ability to adjust a spatial offset δ between the first pattern and the second pattern. The spatial offset δ corresponds to a distance between the respective rows of first and second holes. FIG. 4B shows two different structures, respectively corresponding to two different offsets. The spatial offset δ allows the relative position of the second pattern with respect to the first pattern to be defined. Use of two patterns that are offset with respect to each other allows a fine adjustment of the resonant wavelength. Specifically, the resonant wavelength depends on the relative position of the first and second patterns.

It is believed that such a double-period structure allows high quality factors, typically of the order of 1000, i.e. a full width at half maximum of the peak of 1 nm for a resonant wavelength of 1000 nm, to be obtained.

The modeled curves in FIG. 4A were obtained considering:

-   -   a period a ranging from 1.47 μm to 2.94 μm;     -   a first radius R₁ equal 0.4 times a;     -   a second radius R₂ equal to 0.8×R₁;     -   a thin-layer thickness of 1.47 μm.

The quality factor depends on the ratio R₂/R₁. When R₂/R₁ is of the order of 0.6, the quality factor is equal to a few tens. When R₂/R₁ tends toward 1, the quality factor rapidly increases. The quality factor may then exceed a several thousand.

Generally speaking, the first radius R₁ is smaller than a/2. The ratio R₂/R₁ is preferably higher than or equal to 0.8. The period a may vary between 100 nm and 5 μm, as indicated above.

FIG. 4C shows a variation in the resonant wavelength (y-axis, in nm) of various photonic crystals such as shown in FIG. 4B. In FIG. 4C, the x-axis is an identifier of each photonic crystal in question. Each double-headed arrow covers photonic crystals of the same period a, the latter varying between 288 nm and 304 nm. It may be seen that the period a influences the resonant wavelength. Points encompassed by a given period a correspond to various spatial offsets δ between the two patterns of the photonic crystal. The spatial offset values δ in question are 0 nm, 16 nm, 23 nm, 28 nm, 33 nm and 37 nm, respectively. When the spatial offset δ is equal to 0, the distance between a second hole and each first hole adjacent thereto is equal to a. It has been estimated that a spatial offset δ of 16 nm induces a variation in resonant wavelength of 2 nm. It will be noted that the offset in resonant wavelength (spectral offset) dλ_(k) between two adjacent photonic crystals does not vary linearly with the spatial offset δ. FIG. 4C addresses resonant wavelengths in the very-near-infrared range, i.e. wavelengths comprised between 920 nm and 980 nm. However, these results may be transposed to other infrared spectral bands, and notably between 2 μm and 15 μm. Specifically, disregarding the variation in refractive index, the spectral range of resonance of the photonic crystals is directly proportional to the dimensions of the photonic crystals. If all the dimensions (period, radii, spatial offsets) mentioned with reference to FIG. 4C are multiplied, resonant wavelengths close to 9000 nm, i.e. about 9 μm, are obtained.

Thus, another advantage of the double-period structure such as described above is to allow the resonant wavelength to be adjusted:

-   -   by modifying the period a of the patterns, this leading to a         relatively large variation in the resonant wavelength, typically         10 nm when the period a is varied by 4 nm;     -   by changing the spatial offset δ of both patterns, this leading         to a smaller variation in the resonant wavelength, typically 2         nm for a spatial offset δ of 16 nm, in the particular example of         geometry described with reference to FIGS. 4B and 4C.

The quantitative values given in the previous paragraph were obtained by modeling.

The inventors have fabricated a resonant holder so as to carry out experimental tests, in a spectral band comprised between 920 nm and 980 nm. The characteristics of the resonant holder were as follows:

-   -   thin layer 22: silicon—thickness 58 nm;     -   diameter D of each photonic crystal: 100 μm;     -   radius R₁ of the first holes 23 ₁: a/3;     -   radius R₂ of the second holes 23 ₂: 0.9 R₁:     -   period a: variable between 288 nm and 304 nm, with period         increments Aa of 4 nm;     -   spatial offset of the first pattern with respect to the second         pattern: variable between 0 nm, 16 nm, 23 nm, 28 nm, 33 nm and         37 nm. Thus, for a given period a, 6 photonic crystals were able         to be obtained, each array respectively having a spatial offset         δ chosen from 16 nm, 23 nm, 28 nm, 33 nm and 37 nm.

FIG. 4D shows a variation in the modeled reflectance of a photonic crystal (y-axis) as a function of wavelength (x-axis). The modeled photonic crystal was such as described with reference to FIG. 4B. It had the following geometric characteristics:

-   -   thin layer 22: silicon—thickness 1.47 μm;     -   radius R₁ of the first holes 23 ₁: 0.94 μm;     -   radius R₂ of the second holes 23 ₂: 0.9 R₁.

A resonant wavelength, in reflection, at about 7.8 μm with a quality factor of 1000 may clearly be seen.

FIGS. 5A and 5B are views from above of two examples of photonic crystals, having the same period and two different spatial offsets.

The main steps of fabrication of the photonic crystals are described below, with reference to FIGS. 6A to 6M.

FIG. 6A shows a silicon-on-insulator (SOI) substrate, comprising an upper layer 22 of Si, of 220 nm thickness, lying on an intermediate layer 24 of SiO₂, called the buried oxide (BOX), of 2 μm thickness, the latter lying on a carrier layer 25 of Si of 725 μm thickness.

In order to obtain an upper layer 22 of Si having the desired thickness, said layer undergoes a thermal oxidation, then a chemical etch in hydrofluoric acid to remove the resulting SiO₂ so as to obtain a layer 22 of Si of 58 nm thickness and a layer 22′ of SiO₂ of 80 nm thickness on the upper layer 22. This step is schematically shown in FIG. 6B.

FIG. 6C schematically shows an application of a photoresist R of 570 nm thickness to the oxidized layer 22′. The resist is then exposed to UV through a photolithography mask. Following the exposure, the oxidized layer 22′ and the upper layer 22 are subjected to reactive ion etching (RIE), this step being shown in FIG. 6D. This step makes it possible to form photolithography marks 19, which are referred to as first marks PM, allowing subsequent photolithography to be aligned with respect to the first marks. The resist is then removed, see FIG. 6E.

The steps shown in FIGS. 6F to 6H aim to form holes 23 in the upper layer 22, in periodic patterns such as described above, so as to form the photonic crystals 16 _(k). A photoresist R′, of 300 nm thickness, is deposited on the oxidized layer 22′ (see FIG. 6F). The photoresist comprises a bottom anti-reflective coating (BARC). Etching regions are defined, in the resist R′, via UV exposure through a mask (see FIG. 6G). The holes 23 are formed through the oxidized layer 22′ and the upper layer 22 by RIE, then the resist is removed (see FIG. 6H).

The steps shown in FIGS. 6I to 6M are steps in which the back side of the substrate is etched. A resist R″ is applied to the oxidized layer 22′, the thickness being 1 μm (see FIG. 6I). The applied resist allows the front side of the substrate to be protected, so as to allow the carrier layer 25 to be thinned, decreasing its thickness from 725 μm to 500 μm. The substrate is then flipped and a layer of silicon oxide 26, which acts as a hard mask, is added on the carrier layer (FIG. 6J). A resist R′″ is applied to the layer 26, with a thickness of 5 μm (see FIG. 6K). The resist R′″ is exposed through a mask, so as to define etching regions for the formation of cavities 27 in the back side of the substrate. The substrate is then etched (deep RIE) from the back side, through the carrier layer 25, the intermediate layer 24 acting as an etch-stop layer. FIG. 6L shows the step of etching from the back side. This step allows cavities to be produced facing the holes 23 formed through the layers 22 and 22′. This step allows formation of photonic crystals 16 _(k) that are separate from one other, and borne by the intermediate layer 24, the latter forming a membrane below each photonic crystal. The diameter or the diagonal D of each membrane may be comprised between 50 μm and 500 μm. It is for example equal to 100 μm. The resist R″ is then removed (FIG. 6M).

In the fabricating process described above, the holes 23 are formed by photolithography then etching, this allowing wafer-level fabrication of photonic crystals on the holder. Such a process makes it possible to simultaneously obtain a high number of photonic crystals, this being advantageous from an economic point of view. Alternatively, the holes may be formed by scanning with an electronic beam (e-beam), this making it possible to obtain more precise geometries, to the detriment of the cost and rate of fabrication.

Trials have been carried out to characterize the resonant holder 15 thus formed, and more precisely the photonic crystals 16 _(k). To this end, a drop of water was placed on a resonant holder 15, and a glass slide of 17 μm thickness applied to confine the drop of water. A light-emitting diode centered on 940 nm was used as light source, the spectral band of illumination extending to ±40 nm with respect to the central wavelength of 940 nm. The transmission of the photonic crystals was then characterized via transmission spectrometry tests.

FIG. 7A shows a spectral transmission function of a photonic crystal. The y-axis corresponds to the percentage of light reflected by the photonic crystal, the x-axis corresponding to wavelength. A resonant wavelength below 950 nm corresponding to a Fano resonance may clearly be seen.

For various fabricated photonic crystals 16 _(k), other tests, in fact reflectometry tests, made it possible to establish a resonant wavelength. FIG. 7B is a comparison between modeled resonant wavelengths (black dots) and experimental resonant wavelengths (gray dots). In FIG. 7B, the y-axis corresponds to wavelength (nm) whereas the x-axis designates the reference of the tested photonic crystals. Despite the presence of a substantially constant difference between the modeled and measured resonant wavelengths, it may be seen that it is possible to provide a series of adjacently placed photonic crystals, in the present case 32 adjacently placed photonic crystals, between which the spectral offset of the resonant wavelength dλ_(k) is on average of the order of 2 nm, between 880 nm and 940 nm.

Subject to adjustment of the dimensions of the photonic crystals, and in particular of the period a, of the radii R₁ and R₂, and of the thickness of the thin layer 22 in which the photonic crystals are formed, the results described with reference to FIG. 7B may be transposed to spectral bands comprised between 2 μm and 20 μm.

A resonant holder comprising such photonic crystals allows a resonant wavelength to be identified in a relatively wide spectral band, of spectral width typically larger than 50 nm, and in this example of spectral width equal to 60 nm. When the photonic crystals are aligned, as shown in FIG. 2A or 3A, and coupled to an image sensor, the obtained image allows the photonic crystal transmitting a maximum light intensity to be identified. Thus, as indicated above, the device operates analogously to a spectrometer, the spectral splitting function being performed by the resonant holder 15.

In one advantageous configuration (shown in FIG. 8A) forming a variant of the configuration shown in FIG. 1B, an upstream polarizer 17 ₁ is placed between the light source and the sample, so as to polarize the light wave propagating toward the sample in a direction not parallel to the axes of periodicity of the photonic crystals. The direction of polarization of the upstream polarizer may for example be 45° with respect to the axis Y along which the period of each photonic crystal is repeated, as shown in FIG. 8B. A downstream polarizer 17 ₂ is also placed between the resonant holder and the image sensor. The direction of polarization of the downstream polarizer is oriented 90° with respect to the direction of polarization of the upstream polarizer, this allowing so-called cross-polarization detection. FIG. 8B shows, via a dashed line, the respective polarization directions of the upstream polarizer 17 ₁ and of the downstream polarizer 17 ₂. The addition of such polarizers allows most of the off-resonance light propagating toward the image sensor to be removed. This allows the signal-to-noise ratio of the intensity of the light propagating toward the image sensor at a resonant wavelength to be increased.

Provision may be made to use a cross polarization, combining an upstream polarizer, upstream of the sample, and a downstream polarizer, downstream of the sample, in the embodiments shown in FIGS. 1A and 1B (transmission configurations) and FIG. 1C (reflection configuration).

FIGS. 9A to 9C illustrate an implementation of a holder such as described above, in the cross-polarization configuration. In this figure, the resonant holder is two-dimensional, as described with reference to FIG. 3A. No functionalization was carried out. Photonic crystals aligned in the same column have the same resonant wavelength. FIG. 9A shows an image of the holder placed in contact with water. This figure shows the ability of the device to form an image representative of the spectrum of the illuminating light wave 11. FIG. 9A thus shows the spectral illumination function ƒ discretized according to the various resonant wavelengths λ_(k) of the photonic crystals 16 _(k). FIG. 9B shows an image of the holder placed in contact with ethanol. It also shows the spectral illumination function ƒ discretized according to the various resonant wavelengths of the photonic crystals, said wavelengths having shifted under the effect of the variation in the index of the sample. FIGS. 9A and 9B illustrate the ability of photonic crystals, such as described above, to convert information of spectral nature into spatial information, the latter being able to be read from the image acquired by the image sensor.

Because of the variation in index between water and ethanol, the wavelength of each photonic crystal undergoes a spectral variation δλ, in the present case an increase, this resulting in a slight shift, to the left in the image, of the discretized spectral illumination function. FIG. 9C shows intensity profiles respectively established along a row of photonic crystals of FIG. 9A (gray) and FIG. 9B (black). These profiles allow the spectral variation δλ to be observed. If FIG. 9A forms a reference image, determining the spectral variation δλ allows the variation in index between water and ethanol to be estimated.

According to a second embodiment, the ability of the invention to determine, on the basis of a measurement image, an absorption wavelength λ_(a) of an analyte, is exploited.

In this embodiment, the sample comprises an analyte with a spectral band of absorption Δλ_(a) lying in the spectral band of resonance Δλ_(r) and in the spectral band of illumination Δλ.

As shown in FIGS. 1A to 1C, the sample 20 is placed between the light source and the resonant holder 15. In contrast to the first embodiment, the sample does not necessarily make contact with the resonant holder 15, but it is preferably placed near the resonant holder 15. The image sensor may be placed in either of the transmission configurations shown in FIGS. 1A and 1B, or in the reflection configuration shown in FIG. 1C.

Just as in the first embodiment, the resonant holder 15 comprises various photonic crystals 16 _(k), each photonic crystal having a resonant wavelength λ_(k) in transmission. Preferably, as in the first embodiment, the photonic crystals 16 _(k) are aligned in the same row, for example parallel to the longitudinal axis X, such that the resonant wavelength λ_(k) gradually increases (or decreases) from one photonic crystal 16 _(k), 16 _(k+1) to the next, along the longitudinal axis X.

As shown in FIGS. 10A to 10D, the photonic crystals may be arranged parallel in a column, along a lateral axis Y that is secant with the longitudinal axis X, the longitudinal axis X and lateral axis Y preferably being perpendicular. In one such configuration, the photonic crystals of the same column have the same resonant wavelength λ_(k).

In this example, the structure of the photonic crystals 16 _(k) is such as described in the first embodiment, and more precisely with reference to FIGS. 4B, 4C, 5A and 5B.

As previously mentioned, the various resonant wavelengths λ_(k) of the various photonic crystals define a spectral band of resonance Δλ_(r).

As described above, the sample may be a gas, in which case the analyte is a gas species. The gas may be for example air, or a gas exhaled by a living being, or a gas employed in an industrial process.

The sample may be a solid or a liquid, in which case the analyte is a molecule. Specifically, it is known that the spectral transmission function of light by a sample varies depending on the composition of the latter, because of the presence of absorption peaks corresponding to the vibrational modes of the molecules from which the sample is made up. The presence of absorption peaks is the basis of vibrational spectrometry methods such as infrared spectroscopy or Raman spectrometry.

By transmission function tr(λ), what is meant is a ratio between:

-   -   an intensity i(λ), of a light wave having propagated through the         sample 20, and detected by the image sensor 30, at the         wavelength λ;     -   an intensity, referred to as the reference intensity i_(ref)(λ),         of a light wave detected by the image sensor, at the same         wavelength λ, in the absence of sample.

According to the Beer-Lambert law:

$\begin{matrix} {{t{r(\lambda)}} = \frac{i(\lambda)}{i_{ref}(\lambda)}} & (1) \end{matrix}$

The absorbance abs(λ), at the wavelength λ, is obtained according to the expression:

$\begin{matrix} {{{abs}(\lambda)} = {{{- t}{r(\lambda)}} = {- {\ln\left( \frac{i(\lambda)}{i_{ref}(\lambda)} \right)}}}} & (2) \end{matrix}$

According to this embodiment, the spectral illumination function ƒ may be variable in the spectral band of resonance, or constant in the spectral band of resonance Δλ_(r).

In the examples schematically shown in FIGS. 10A to 10D, the illumination function ƒ is assumed to be constant in the spectral band of resonance: the photonic crystals are illuminated uniformly. In each of these figures, images of the same resonant holder 20 have been shown. Each image comprises regions of interest ROI_(k) that are optically coupled to a photonic crystal 16 _(k) the resonant wavelength of which is λ_(k).

FIG. 10A corresponds to a reference image, obtained without analyte. The signal is of uniform intensity in the various regions of interest, since it is not attenuated by an analyte. Each row represents a spectral intensity of the illuminating light wave 11, discretized according to the various resonant wavelengths λ_(k). Each column represents photonic crystals having the same resonant wavelength. Thus, with each column are respectively associated the resonant wavelengths λ_(k1), λ_(k2), λ_(k3) and λ_(k4).

FIG. 10B corresponds to a measurement image acquired by placing, between the light source 10 and the resonant holder 15, a sample comprising an analyte the spectral band of absorption Δλ_(a) of which comprises an absorption wavelength λ_(a) corresponding to the resonant wavelength λ_(k1). The absorption of the illuminating light wave 11 results in a relative darkening of the regions of interest ROI_(k1), representing the photonic crystals the resonant wavelength of which is λ_(k1).

FIG. 10C corresponds to a measurement image acquired by placing, between the light source 10 and the resonant holder 15, a sample comprising an analyte the spectral band of absorption Δλ_(a) of which comprises an absorption wavelength λ_(a) corresponding to the resonant wavelength λ_(k3). The absorption of the illuminating light wave 11 results in a relative darkening of the regions of interest ROI_(k3), representing the photonic crystals the resonant wavelength of which is λ_(k3).

FIG. 10D corresponds to a measurement image acquired by placing, between the light source 10 and the resonant holder 15, a sample comprising analytes the spectral band of absorption Δλ_(a) of which comprises resonant wavelengths λ_(k1) and λ_(k3). The absorption of the illuminating light wave 11 results in a simultaneous darkening of the regions of interest ROI_(k1) and ROI_(k3), representing the photonic crystals the resonant wavelengths of which are λ_(k1) and λ_(k3).

FIGS. 10E and 10F schematically show profiles of an intensity of the light transmitted by photonic crystals in the configurations illustrated in FIGS. 10A and 10D, respectively. In these figures, the y-axis corresponds to an intensity, whereas the x-axis represents the position of the photonic crystals, according to their respective resonant wavelength.

In the measurement images, the intensity of the regions of interest corresponding to the photonic crystals the resonant wavelength of which is in the spectral band of absorption of an analyte depends on the absorption of the illuminating light wave by the analyte, as expressed by expression (2). The absorption in particular depends on an absorption coefficient of the analyte, said coefficient varying as a function of wavelength, and of the amount of analyte present in the sample. Since the value of the absorption coefficient, in the various spectral bands of resonance, is known, an estimate of the attenuation allows the amount of analyte present in the sample to be estimated.

The measurement image allows the intensity i (λ_(a)) of the light wave having propagated through the sample, in the presence of analyte, at an absorption wavelength λ_(a), to be estimated. The absorption wavelength is a resonant wavelength λ_(r) of a photonic crystal belonging to the spectral band of absorption Δλ_(a). The reference intensity i_(ref)(λ_(a)) of the light wave having propagated through the sample, in the absence of analyte, at the absorption wavelength, may be estimated on the basis of a reference image acquired in the absence of a sample containing the analyte.

Alternatively, the reference intensity i_(ref)(λ_(a)) may be estimated from the image acquired by the image sensor, in the presence of the sample, on the basis of a region of interest ROI_(k) optically coupled to a photonic crystal 16 _(k) the resonant wavelength λ₀ of which is considered to not be absorbed by the analyte. Knowledge of the illumination function ƒ makes it possible to pass from an intensity measured at the unabsorbed wavelength λ₀, to the reference intensity i_(ref)(λ_(a)) at the absorption wavelength λ_(a).

The examples described with reference to FIGS. 10A to 10D illustrate the ability of a resonant holder, such as described above, to obtain a wavelength spectrum of the light wave having propagated through the sample. In this embodiment, use is made of the fact that this spectrum comprises the signature of analytes present in the sample that exhibit a significant absorption at one or more resonant wavelengths λ_(k) in the resonant range Δλ_(r). As mentioned with reference to the first embodiment, the spectral resolution depends on a discretization pitch dλ_(k) representing an offset between the resonant wavelengths of two adjacent photonic crystals 16 _(k), 16 _(k+1) of the holder. This discretization pitch may be very small, and is typically smaller than 10 nm. This allows a precise identification of the resonant wavelengths at which an absorption of the illuminating light wave is observed. As a result, the analyte that caused the absorption may be accurately identified. Comparison with a reference measurement also allows the amount of analyte to be quantified.

When the sample is a gas, a pre-concentrating loop may be placed upstream of the device. As known, such a loop comprises an absorbent medium able to reversibly trap one or more gas species flowing through the loop. This allows reversible storage of an analyte. Prior to the measurement, the absorbent media is configured to release the trapped gas species. The use of such a pre-concentrating loop allows the amount of analyte between the light source and the resonant holder of the device to be increased.

Regardless of the embodiment, the sample is preferably spatially homogeneous. By spatially homogeneous, what is meant is that, at least in one row of photonic crystals, the amount of analyte in the sample is considered to be constant.

The invention will possibly be used to detect and quantify an analyte in samples, for example in the field of biological analysis or to assist with medical diagnosis. The invention may also be employed to monitor industrial processes, for example in the food industry, or even to monitor the environment. 

1-25. (canceled)
 26. A method for analyzing whether a sample includes an analyte, the sample lying on a resonant holder, the resonant holder comprising a surface on which lie a plurality of photonic crystals, which are separate from one another, and the sample being disposed between a light source and the photonic crystals, wherein: one resonant wavelength is associated with each photonic crystal, the resonant wavelengths of the photonic crystals defining a resonance spectral band lying between 2 μm and 20 μm; and transmission or reflection of light by each photonic crystal is maximum at the an associated resonant wavelength; the method comprising: a) illuminating the resonant holder with the light source, the light source emitting an illuminating light wave that defines an illumination spectral band that at least partially covers the resonance spectral band, wherein a plurality of photonic crystals are simultaneously illuminated; b) acquiring, with an image sensor, a measurement image of the resonant holder, the measurement image comprising different regions of interest, each region of interest being optically coupled to one photonic crystal such that each region of interest represents an intensity transmitted or reflected by each photonic crystal, while the resonant holder is illuminated; and then determining whether the analyte is present in the sample on the basis of the measurement image; wherein each photonic crystal comprises: first holes formed in a thin layer and having a first dimension, the first dimension being a radius or a diagonal, the first holes defining a first periodic pattern; second holes formed in the thin layer and having a second dimension, the second dimension being strictly smaller than the first dimension, the second holes defining a second periodic pattern; the second pattern and the first pattern are offset with respect to each other, parallel to the surface of the resonant holder, by a spatial offset, the spatial offset being variable between at least two different photonic crystals; and the resonant wavelength associated with each photonic crystal depends on the first dimension, on the second dimension, and on the spatial offset.
 27. The method of claim 26, further comprising: c) using a reference image, the reference image being representative of an image acquired by the image sensor when the resonant holder is illuminated in the illumination spectral band in a reference configuration, in an absence of analyte; d) comparing the measurement image, acquired in b), with the reference image, in c); and e) based on the comparison, determining whether the analyte is present in the sample.
 28. The method of claim 27, comprising, prior to a), forming the reference image, comprising: illuminating the resonant holder with the light source, in the illumination spectral band, without analyte between the light source and the resonant holder; and acquiring an image of the resonant holder with the image sensor, the image thus acquired forming the reference image.
 29. The method of claim 27, comprising obtaining the reference image by: illuminating, in the illumination spectral band, a reference holder that is considered to be representative of the resonant holder illuminated in a), an amount of analyte between the reference holder and the light source being considered to be zero; and forming an image of the reference holder, the image thus formed corresponding to the reference image.
 30. The method of claim 27, wherein the resonant holder comprises reference photonic crystals the resonance spectral band of which is one of: not modified by the analyte, and corresponds to a wavelength not absorbed by the sample; and wherein the reference image is an image of the reference photonic crystals when they are illuminated in the illumination spectral band.
 31. The method of claim 30, wherein the reference image and the measurement image form two parts of the same image acquired by the image sensor.
 32. The method of claim 27, wherein: the sample is a liquid; the resonant holder makes contact with the sample; the resonant holder comprises photonic crystals addressing the analyte, which are configured to make contact with the analyte; the measurement image comprises regions of interest that are associated with the analyte and optically coupled to the photonic crystals addressing the analyte; under an effect of contact with the analyte, the resonant wavelength of the photonic crystals addressing the analyte undergoes a spectral variation; and the intensity of the illumination varies, in the resonance spectral band, according to a spectral illumination function.
 33. The method of claim 32, wherein, for each photonic crystal addressing the analyte, the resonant wavelength depends on a refractive index of the sample at an interface between the sample and the photonic crystal, the refractive index varying as a function of an amount of analyte making contact with the photonic crystal.
 34. The method of claim 32, wherein e) comprises: on a basis of the measurement image, determining a measurement profile of the intensity of the regions of interest associated with the analyte; and on a basis of the reference image, determining a reference profile representative of the intensity in absence of analyte or in a presence of a known amount of analyte in the sample, of the regions of interest associated with the analyte, wherein whether the analyte is present is determined depending on a shift of the measurement profile with respect to the reference profile.
 35. The method of claim 34, wherein the amount of analyte is estimated depending on the shift of the measurement profile with respect to the reference profile.
 36. The method of claim 32, wherein e) comprises: in the measurement image, determining a measurement position of a region of interest exhibiting a maximum intensity value among the regions of interest associated with the analyte; and in the reference image, determining a reference position of a region of interest exhibiting a maximum intensity value among the regions of interest associated with the analyte, wherein the presence of the analyte is determined depending on a shift of the measurement position with respect to the reference position.
 37. The method of claim 36, wherein an amount of analyte is estimated depending on the shift of the measurement position with respect to the reference position.
 38. The method of claim 32, wherein e) further comprises estimating an amount of analyte in the sample, depending on the comparison between the measurement image and the reference image.
 39. The method of claim 26, wherein the photonic crystals lie in a holder plane, the illuminating light wave propagating to the resonant holder parallel to a propagation axis that is perpendicular or substantially perpendicular to the holder plane.
 40. The method of claim 26, wherein the resonant holder is disposed between the light source and the image sensor, such that, in b), each region of interest formed in the measurement image is representative of an intensity transmitted by the photonic crystal to which said region of interest is optically coupled.
 41. The method of claim 26, wherein: the resonant holder bounds a half-space comprising the light source; and the image sensor is disposed in the half-space, such that each region of interest formed in the measurement image is representative of an intensity reflected by the photonic crystal to which said region of interest is optically coupled.
 42. The method of claim 26, wherein: the sample is disposed between the light source and the resonant holder; the analyte is able to absorb the illuminating light wave, emitted by the light source, in an absorption spectral band; and the absorption spectral band comprises at least one absorption wavelength corresponding to a resonant wavelength of a photonic crystal, the method comprising, on the basis of the measurement image: determining at least one photonic crystal, the resonant wavelength of which corresponds to the absorption wavelength; identifying the resonant wavelength of said photonic crystal; and determining whether the analyte is present on a basis of the resonant wavelength of the photonic crystal thus determined.
 43. The method of claim 42, comprising: on a basis of the measurement image, estimating an absorption light intensity, transmitted or reflected by at least one photonic crystal the resonant wavelength of which is comprised in the absorption spectral band; estimating a reference light intensity, transmitted or reflected by said photonic crystal, in the absence of analyte; and estimating an amount of analyte on a basis of the absorption light intensity and of the reference light intensity.
 44. The method of claim 26, wherein the sample is one of a gas, liquid and solid.
 45. A device for detecting an analyte within a sample, comprising: a light source; an image sensor; and a resonant holder disposed between the light source and the image sensor, wherein the image sensor is configured to acquire an image of the resonant holder, the resonant holder is configured to be placed in contact with a sample, and the resonant holder comprises an area containing photonic crystals, which are separate from one another, wherein: the light source is configured to simultaneously illuminate a plurality of photonic crystals in an illumination spectral band lying between 2 μm and 20 μm; one resonant wavelength is associated with each photonic crystal, wherein transmission or reflection of the light by each photonic crystal is maximum at an associated resonant wavelength; at least two photonic crystals are configured to make contact with the analyte, said at least two photonic crystals being associated with two different resonant wavelengths, the resonant wavelength associated with each photonic crystal depending on an amount of analyte in contact with the photonic crystal; each photonic crystal comprises: first holes formed in a thin layer and have a first dimension, wherein the first dimension is a radius or a diagonal and the first holes define a first pattern; second holes formed in the thin layer and that have a second dimension, wherein the second dimension is strictly smaller than the first dimension and the second holes define a second periodic pattern; and the second pattern and the first pattern are offset with respect to each other, parallel to the surface of the holder, by a spatial offset that is variable between at least two different photonic crystals; and the resonant wavelength associated with each photonic crystal depends on the first dimension, on the second dimension, and on the spatial offset.
 46. The device of claim 45, wherein: the photonic crystals are aligned parallel to a longitudinal axis; and the resonant wavelengths respectively associated with two adjacent photonic crystals are offset by a discretization pitch comprised between 1 nm and 50 nm.
 47. The device of claim 45, wherein: a plurality of photonic crystals are aligned in a row parallel to a longitudinal axis; and the resonant wavelength respectively associated with a photonic crystal gradually increases, or decreases, along the longitudinal axis.
 48. The device of claim 45, wherein: the resonant holder comprises a plurality of rows of photonic crystals parallel to one another, and the photonic crystals form columns, parallel to a lateral axis, such that the photonic crystals of a given column have a same resonant wavelength.
 49. The device of claim 45, wherein the photonic crystals addressing the analyte are covered with a functionalization layer configured to selectively capture the analyte on the photonic crystals.
 50. The device of claim 45, wherein the resonant holder comprises: photonic crystals of rank 1, which are configured to capture a first analyte; and photonic crystals of rank j, which are configured to capture a jth analyte different from analytes captured by the photonic crystals of rank lower than j. 